Permanent magnet (“PM”) electromechanical machines utilize permanent magnets to convert rotational inputs into electricity or electrical inputs into rotational motion. One example is wind power units (WPUs) that generate electricity from the energy in wind. Generally, a PM generator or motor has three components. A first component, a stator, is a cylindrical housing that contains electrical windings that remain immobile during electricity generation. A second component, a rotor, is a rotatable assembly containing permanent magnets that spin with respect to the stator. The relative movement between the rotor and the stator produces a moving magnetic field, which induces an electrical current in the stator electrical windings, thereby producing electricity. A third component, such as an axle, rotationally supports the rotor with respect to the stator, enabling the two to rotate relative to each other.
As with other electromechanical machines that use permanent magnets, the permanent magnets in WPUs are typically installed relatively early in the assembly process. While it may be convenient to install magnets early in the assembly process, the presence of high strength permanent magnets can make later stages of assembly and installation inconvenient and even dangerous. This situation arises because subsequent assembly and installation steps require use of, and transportation near, ferromagnetic materials that are strongly attracted to the permanent magnets. Further, the process of assembling the stator and the rotor can be more difficult because of the tight spacing at the machine airgap, and the high magnetic forces between the rotor and the stator. Such assembly of magnets can especially complicate field repair and service of WPUs due to remote locations and positioning at the top of high towers.
Permanent magnets are often fastened within PM machines using bolts or other similar mechanical means directly secured to the magnets. While bolting the magnet to the rotor does securely fasten the magnet, bolting also makes removal of the magnet during maintenance difficult by, for example, requiring disassembly of the generator in order to remove the bolts. Directly bolting the magnets may also remove magnetic material. Removal of magnetic material can change the magnetic flux characteristics, thereby altering electricity generation. Furthermore, removing magnetic material and using bolts risks damaging the magnet during generator assembly or maintenance because of the stresses exerted on the magnet. These factors increase the effort and expense required to maintain a PM electromechanical machine, especially a WPU located in the field.
Another challenge in the design of PM machines is temperature control. As is well known in the art, managing magnet temperature is important, among other reasons, to protect against demagnetization during machine operation, and to allow the use of lower cost magnets. For example, a ten degree Celsius decrease in magnet operating temperature may provide a reduction in magnet cost in the range of about $25,000 for a large, permanent magnet machine utilizing rare earth alloy magnets. While many cooling schemes have been proposed, often they are complex or ineffective in practice. But even when a cooling scheme is effective to an extent, further efficiencies still can be achieved by additional temperature control means if such means can be cost effectively incorporated into the design without adding undue complexity.